Bone implants and grafts – they’ve been around for years, but are still a very problematic issue. For an implant has to accepted by the body (and not result in an inflammatory response), widely available and easy to use in a surgical environmen, and they need to be cost-effective. Nowadays putty-type fillers, custom scaffolding and even salvaged cadaver bones are regularly used, but they can still cause inflammatory reactions and are particularly unsuited for underage patients – who can expect multiple surgeries over the years as they mature. Even then, these options are usually more brittle than the real thing.

Fortunately, 3D bioprinting could offer a solution by building environments in which bone cells can be encouraged to form completely new structures. The big challenge right there is finding the materials that can be 3D printed into scaffolding, in which stem cells can grow and stay alive long enough to be implanted. Several research team from all around the world are working on this material challenge, and just a few months ago a team from the University of Bristol announced their successes with a 3D printable seaweed-based material that is particularly suited for cartilage growth.

But an even more widely applicable solution has just appeared out of the labs at Northwestern University. Called hyperelastic bone (or HB), it could be the breakthrough the world has been waiting for. This 3D printable hyperelastic bone bioink can be turned into bone implants of any size, shape and form (even entire skulls are possible), and can be implanted into the body. Most importantly, the material is ultra-elastic and robust, allowing the doctors to manipulate it in any surgical setting and ensure that implants have the exact shape necessary. Once inside, blood vessels and cellular structures quickly fill up the scaffold and enable bone regeneration.

Shah is no stranger to 3D printing. The assistant professor has previously pioneered and assisted in various 3D printable material breakthroughs. Back in 2015, she led a team during the development of a 3D printable graphene ink for electronic and biomedical applications, among others. As a mentor, she was also involved in the development of 3D printed clean energy fuel cells at Northwestern.

Thanks to hyperelastic bone’s very appealing biomedical properties, this could be her biggest 3D printing achievement so far. As Shah revealed, even her team was surprised by the material’s amazing elasticity. “The first time that we actually 3D printed this material, we were very surprised to find that when we squeezed or deformed it, it bounced right back to its original shape,” she said. “Our vision is to have 3D printers in a hospital setting where we provide the hyperelastic bone ink, so surgeons can make individual implants within 24 hours. You could make off-the-shelf, or patient-specific implants using scans from patients.”

The hyperelastic bone implants themselves largely consist of a naturally occurring mineral called hydroxyapatite – a form of calcium readily found in the human bone. It is also regularly used during reconstructive surgeries already, but suffers from extreme brittleness. When mixed with a polymer, however, it becomes hugely flexible and easily 3D printable. But 90 percent of the implants are still made from this calcium material, ensuring that the implants closely mimics existing bone in terms of pore structure and mineral content, which enhances the body’s ability to regenerate actual bone. “Cells can sense the hydroxyapatite and respond to its bioactivity,” Shah said. “When you put stem cells on our scaffolds, they turn into bone cells and start to up-regulate their expression of bone specific genes. This is in the absence of any other osteo-inducing substances. It’s just the interaction between the cells and the material itself.”

But more importantly, this flexibility makes it a fantastic option for underage patients with bone defects. “Adults have more options when it comes to implants,” Shah said. “Pediatric patients do not. If you give them a permanent implant, you have to do more surgeries in the future as they grow. They might face years of difficulty.” HB’s material properties, in contrast, provides far more flexibility during rehabilitation and growth.

The benefits don’t end there. For hyperelastic bone is 3D printed at room temperature, enabling doctors to incorporate various other materials into the ink – even antibiotics. “We can incorporate antibiotics to reduce the possibility of infection after surgery,” Shah said. “We also can combine the ink with different types of growth factors, if needed, to further enhance regeneration. It’s really a multi-functional material.”

But before we get too optimistic, 3D printed hyperelastic bone still needs to be tested on humans. So far, the researchers evaluated the material during animal testing, with the results appearing in the Science Translational Medicine Article. Among others, they saw that their 3D printed implants healed spinal defects in rats at a very fast pace – rivalling the results of existing treatments. They also healed a damaged skull of a rhesus macaque with the material, a process that also showcased a surgeon’s ability to perfectly cut the implants to size.

Clinical testing on humans could follow within five years. “That could change the world of craniofacial and orthopaedic surgery, and, I hope, will improve patient outcomes,” Shah argued. However, laboratory results were already promising, as they placed human stem cells into the scaffolds during various tests. Not only did the cells grow without any problems, they also began producing their own bone minerals. “It went from a synthetic scaffold to natural minerals being created by the cells themselves,” co-author Adam Jakus argued.

While it could thus take a while before 3D printed hyperelastic bone’s usefulness for human patients becomes fully apparent, these initial studies certainly are promising. Hyperelastic bone is also expected to be quite cheap to produce and can be stored for about a year, making it ideal for use around the world. “You could just ship it way ahead of time, have it on the shelf until it's needed rather than having to create a complex biomaterial that needs to be heavily refrigerated or frozen,” said Jakus. “those types of facilities may not be accessible [in developing countries], so being able to open a package and to use the material is fantastic.” Could this be the breakthrough we have been waiting for?